We are investigating the structure, equilibrium dynamics, and folding of human prolactin. Prolactin is a 199-residue member of the hernatopoietic cytokine family, which includes growth hormone. Prolactin is implicated in a large number of biochemical pathways, but it is named for its role in promoting lactation during pregnancy. X-ray structures of growth hormone andgrowth hormone bound to the extracellular domain of the lactogenic receptor have given insights into the general features of hematopoietic hormone action; the native structure is a four-helix bundle with an unusual up-up-down-down topology, and it acts by inducing dimerization of thereceptor in a two-step sequential binding process. Growth hormone binds to both the somatogenic receptor and to the lactogenic receptor. In contrast, prolactin binds only to the lactogenic receptor. In addition, zinc is known to mediate receptor binding of growth hormone, whereas it does not appear to play a role in prolactin binding. Prolactin has thus far eluded detailed structural analysis, and so the molecular details responsible for the difference in the behavior of growth hormone and prolactin remain unexplained. We are carrying out the determination of the solution structure of prolactin by high resolution multinuclear NMR. We are investigating the folding and stability of prolactin using equilibrium centrifugation, circular dichroism, scanning calorimetry, and hydrogen exchange kinetics. ESI and NLkLDI mass spectrometry are used to determine the protein molecular weight and sites of disulfide linkage.